Microporous membranes based on semi-crystalline polymers have been previously prepared. Most of the commercial membranes of these polymers are symmetric in nature. The production of such microporous membranes are described, for example, in U.S. Pat. No. 4,208,848 for PVDF and in U.S. Pat. No. 4,340,479 for polyamide membranes. These preparations are generally described to consist of the following steps: a) preparation of a specific and well controlled polymer solution, b) casting the polymer solution onto a temporary substrate, c) immersing and coagulating the resulting film of the polymer solution in a nonsolvent, d) removing the temporary substrate and e) drying the resulting microporous membrane.
Polyvinylidene fluoride (PVDF) membranes as described above are made by casting a lacquer in a specific coagulant (e.g. acetone-water mixture, IPA-water mixture or methanol) that allows the formation of a microporous, symmetric membrane. A similar process is used for symmetric polyamide membranes. In these prior art processes, the semi-crystalline polymers used primarily lead to symmetric membranes. Membranes made from such semi-crystalline polymers have a characteristic property whereby the thermal history of the polymer solution prior to casting has a dramatic effect on membrane performance. In general terms, it has been found that the higher the maximum temperature to which the solution is heated to, the larger the rated pore size of the resulting microporous membrane. In one method of controlling pore size, the polymer solution is made at a relatively low temperature in a typical manufacturing stirred tank vessel, or similar, and then heated to the desired maximum temperature by, for example a heated jacket. Variability in lacquer history can therefore cause reduced process yields. It can be appreciated that fine control over the thermal history of a large mass of viscous solution is difficult. In-line heating and cooling treatment is sometimes used in order to provide improved control over the thermal history of the polymer solution being processed. An in-line process provides a means for heating the solution as it is transported through a pipeline, thereby reducing the effective mass of solution being heated. The shorter heating contact time necessitated by in-line heating requires good mixing to obtain even heat treatment. Membranes made from solutions having a uniform thermal history throughout its bulk tend to produce symmetric membranes.
Microporous membranes are described as symmetric or asymmetric. Symmetric membranes have a porous structure with a pore size distribution characterized by an average pore size that is substantially the same through the membrane. In asymmetric membranes, the average pore size varies through the membrane, in general, increasing in size from one surface to the other. Other types of asymmetry are known. For example, those in which the pore size goes through a minimum pore size at a position within the thickness of the membrane. Asymmetric membranes tend to have higher fluxes compared to symmetric membranes of the same rated pore size and thickness. Also, it is well known that asymmetric membranes can be used with the larger pore side facing the fluid stream being filtered, creating a prefiltration effect.
Practitioners have developed complex methods to produce asymmetric membranes from semi-crystalline polymers. PVDF membranes are produced by thermally induced phase separation (TIPS), where the temperature of an extruded film, tube or hollow fiber of a homogeneous polymer solution is quenched down to a lower temperature thereby inducing phase separation. Examples of PVDF membranes made by TIPS are disclosed in U.S. Pat. Nos. 4,666,607, 5,013,339 and 5,489,406. These processes require high temperatures and screw type extruders, increasing process complexity.
U.S. Pat. No. 4,629,563 to Wrasidlo discloses asymmetric membranes that can be characterized by a skinned layer that is relatively dense and thick with a gradually changing pore size beneath the skinned layer. Claimed ratios of pore sizes in opposite surfaces ranges from 10 to 20,000 times. This process requires the use of an “unstable liquid dispersion.” Use of such dispersions reduces the control available over the overall process.
U.S. Pat. Nos. 4,933,081 and 5,834,107 disclose humid air exposure applied to PVDF-PVP solutions to create PVDF membranes to produce microporous membranes with high flux characteristics. By using similar humid-air exposure techniques as in U.S. Pat. No. 4,629,563, some subtle but apparently important differentiations are made from the Wrasidlo patent. These patents teach that differences in lacquer composition and humid air exposures can lead to large structural changes. In U.S. Pat. No. 4,933,081, membranes having hourglass porous structure are produced with the average diameters of the pores decreasing along a line from a microporous surface to a coarse pore surface. Thereafter, the pore size increases again along that same line. Both methods require additional control of the humid atmosphere-polymer solution contact time, humid air velocity, temperature and humidity, thereby increasing process complexity.
Furthermore, U.S. Pat. No. 5,834,107 describes structures having a gradual changing pore size from microporous side to a coarse surface. All the structures also have some large open volumes in portions of the membrane near the coarse surface of the membrane. This structure is defined in the patent as filamentous webs. The large open volumes, although they may be different in origin from macrovoids, can cause similar mechanical failures in membrane application and are therefore not desirable in applications where high integrity is required. The presence of these large open volumes is not beneficial in terms of retention, since the diameter of the flow paths are much too large to retain typical solutes or particles filtered by microporous membranes. In addition, the methods described above always use a high molecular weight additive in the lacquer and humid air exposure.
U.S. Pat. No. 6,013,688 discloses making PVDF membranes that appear to have an isotropic structure, containing a dense array of closely aligned and contiguous polymer particles. A part of the structure is characterized by so-called spherical craters. Such structures tend to be mechanically weak.
U.S. Pat. Nos. 5,626,805 and 5,514,461 disclose a complex thermally induced phase separation technique (TIPS) that quenches both sides of a film of a polymer solution with a different rate to effect different supersaturation in different time frames. The thermal quench technique can lead to asymmetric structures being characterized in cross section by a beady, open structure at one surface and a leafy, more tight structure at the other surface. However, to obtain an improvement in flux, it is not sufficient to have a larger pore size on both surfaces, but also that the pore size changes throughout the membrane.
U.S. Pat. No. 5,444,097 discloses heat induced phase separation for making high flow membranes. This method depends on generating phase separation by heating of a polymer solution to above its lower critical solution temperature (LCST). The LCST is a temperature at which a polymer solution becomes cloudy due to phase separation of the solution. A minimum on a curve of cloud point temperature as a function of polymer concentration is referred to the lower critical solution temperature. This technique is very specific for polymer solutions that are characterized by a lower critical solution temperature (LCST). In this process the polymer solution must be maintained at the desired temperature above the LCST. This increases the complexity of the process because the solvent laden solution must be transported from the heating region to the immersion region of the process while maintaining the desired temperature in a narrow temperature zone above or below the desired temperature so as not to change the final pore size from the designed pore size.
Accordingly, it would be desirable to provide a simple, easily controlled process for forming microporous membranes having asymmetric pore structure wherein the pore size throughout the membrane thickness varies.